1. Field of the Invention
This invention relates to the field of treating asphaltic concrete paving materials and the recycling of said materials. It is necessary to perform the recycling of the paving materials at the least cost per ton of material. Recycling of paving materials must also be done with minimal damage to the asphalt and with minimum air pollution. The use of hot air or high heat combustion gases to heat asphaltic paving materials and the use of microwave fields is well known in the art.
The single greatest cause of inefficiency in heating asphaltic paving is the necessity for driving off water vapor. Until such water vapor is driven off and removed from the paving materials, the materials cannot be heated to their ideal working temperature which is well in excess of the boiling temperature of water. Known methods of reclamation of asphaltic pavement are limited to approximately 50% recycled asphalt material as a practical matter. These methods produce substantial environmental degradation and damage the finished asphaltic product if more than 50% recycled material is used.
In the prior art little concern has been given to efficient heat transfer, heat utilization, and to prevention of degradation of the asphalt while maintaining overall efficiency.
There is no known economically feasible way of recycling 100% reclaimed asphalt pavement (RAP). The system of the prior art either do no work or are not economically viable.
2. Brief Summary of the Invention
This invention is a method of treating a reclaimed asphaltic concrete paving material which utilizes as its first step the removal of loose unwanted fines (small particles)from the reclaimed asphalt paving (RAP). Strictly speaking, we are not removing all fines from the RAP. These are loose fines. There is a substantial portion of fines in the RAP which is bound up by the asphalt and contained in the larger agglomerates of RAP. These agglomerated fines are dry and are needed in the final mix. It has been found that the moisture in the RAP is on the surface of the particles of the material. The small particles have a far greater surface area to volume ratio than the larger particles. The small particles have on their surfaces, the larger portion of the water in the RAP. Therefore, if the fine materials are removed prior to the heating of the RAP, a substantial savings in energy input may be achieved. It is also true that an excess of fines in the mix is unacceptable because it will not meet specifications for gradient of material.
In this invention, an air sorter used to separate out fine materials which are less than a predetermined size. The air sorter produces agitation of the RAP material which breeds small particles and enables them to be carried away. The air used in the initial air sorter may also be heated in order to begin the process of transferring heat to the RAP particles which are to be later reprocessed. The materials which are greater than the predetermined size (remaining RAP) are then first heated in a recirculating air dryer to a first predetermined temperature. The RAP at the first predetermined temperature is then heated to a higher predetermined temperature required for working the asphaltic pavement in a microwave tunnel. The elimination of vapor and moisture and dirt in the RAP layer in the recirculating air dryer makes the heating in the microwave tunnel easier because arcing and heat loss caused by these materials is substantially eliminated. By the use of these three steps, substantial energy savings are achieved by first, eliminating substantial quantities of water through the removal of fines by the air sorter, secondly, by heating the RAP to a first predetermined temperature in a recirculating air dryer which is very efficient. Still further, efficiencies are achieved by the use of the microwave radiation tunnel which allows heating of the RAP without overheating the asphaltic components. Overheating causes smoking and inefficient use of the available heat and degradation of asphalt.
Although microwave has many characteristics that make it a very good method for second stage heating, it has the disadvantage of having a high initial cost and high consumption of expensive electricity. There are several alternatives to microwave for finish heating (second stage heating). The temperature can be raised from 220.degree. F. to a desired ending temperature with an air impingement dryer similar to the one used for the initial sorting and/or heating. Infrared heating may also be used as an alternative to microwave heating. Both infrared and air impingement dryer heating are less expensive than microwave. Both infrared and air impingement heating are slow because they must transmit heat from the surface of the RAP to the interior of the RAP by conduction.
In another embodiment of this invention, the RAP is separated into a plurality of sizes, and each of the sizes is then heated in a separate conveying and heating apparatus. By separating into groups of different sizes, different heating rates or methods may be applied to each group, thereby eliminating the problem of overheating small particles and underheating larger particles. Through particle separation, better control over the asphaltic pavement recycling operation is achieved.
The microwave heater in accordance with this invention may comprise an aluminum or stainless steel enclosed microwave heating tunnel in combination with a microwave transparent conveying belt. Still further, the microwave tunnel may provide for placement of the microwave antenna within twelve inches or more of the RAP which is being treated. The thickness of the RAP layer on the conveyor belt, the height of the wave guide above the top of the RAP, and the spacing of the microwave broadcasting antenna along the length of the tunnel are a function of the frequency of the microwave being used. The spacing of twelve inches from the antenna to the RAP refers to the distance which would be used with 915 mHz waves. The spacing will be approximately 4.5 inches if a 2,450 mHz wave is used. Arcing and undesirable dust clouds can be kept to a minimum in the region of the microwave heating element by placement of a dust shield between the microwave antenna and the RAP passing through said tunnel. The dust shield may be fiberglass cloth or other suitable fabric which is transparent to the microwave.
In the treatment of RAP with a recirculating air dryer, it has been found that the optimum drying and heating is obtained when the RAP on a conveyor is moved to the side sufficiently to expose the belt.
The recirculating air dryer utilizes tubes to provide a warm high velocity air stream to move particles on the conveyor belt. The recirculating air dryer applies heat in the range of 300.degree.-450.degree. F., and is of sufficient velocity so as to move the RAP particles beneath each tube sideways so as to expose a portion of the conveyor belt. The temperature is limited by that which will produce smoking of the RAP at the speed of operation of the conveyor belt. It has been found that the best drying characteristics are achieved when a portion of the belt is exposed by each tube injecting air into the RAP. The point of belt exposure coincides with the velocity which provides the most efficient air drying of the RAP. A plurality of tubes inject air onto the belt, and the RAP assumes a configuration which may be described as wind rows along the conveyor belt. As the wind rows are shifted back and forth, they expose different portions of the aggregate to the hot, high velocity air, thereby improving the drying characteristics.